Not Applicable.
Not Applicable.
1. Field of the Invention
This invention relates to a process and apparatus for thermal desorption of contaminates from a mixture of soil and rocks using desiccated, electrically-heated fresh air to treat the soil and rocks which have been excavated and placed in a thermally conductive treatment container which is then placed in a thermally insulated treatment chamber. The fresh, hot, desiccated air is drawn through the soil treatment container, cooled, and released; or discharged to a treatment system, as required or needed, prior to release to the atmosphere.
2. Description of Related Art
The use of petroleum hydrocarbons as a fuel source is ubiquitous in society. Consequently, petroleum hydrocarbon products are stored and handled in great quantities. One risk associated with the storage and handling of petroleum hydrocarbons is the potential for spillages during handling or the potential for leakage during storage. Due to the negative environmental impact associated with spills and leakages of petroleum hydrocarbons, rules have been established at the local, state and federal levels. These rules primarily focus on preventing petroleum hydrocarbon releases to the environment from occurring. These rules also have provisions that require the responsible party to remediate petroleum hydrocarbon releases to the environment.
In the field of petroleum hydrocarbon remediation from soil, there are two basic approaches: applying a treatment technique to soil in place (in-situ), or applying a treatment technique to excavated soil (ex-situ). There are advantages and disadvantages for each approach and the selection of the approach is based on the site-specific circumstances of each petroleum hydrocarbon release. The present invention deals with an ex-situ remediation technique. Using this invention, soil that is contaminated with petroleum hydrocarbons will require excavation prior to treatment.
There are numerous ex-situ treatment techniques available. The present invention may be categorized as a thermal desorption technology. Another treatment type available may be characterized as liquid solvent, where a liquid is used as the treatment medium and is introduced to the soil and then removed, taking contaminates with it.
Prior art for ex-situ thermal desorption technologies reveal that there are two basic categories of thermal desorption techniques: (1) techniques that involve mechanical agitation of the soil during the heating process and (2) techniques that are applied to a static configuration of soil.
Often the techniques that involve mechanical agitation also operate in a continuous process where the soil is continuously introduced to the process and is mechanically moved through the process apparatus until treatment is complete, and then is continuously discharged to a container for disposal or re-use.
Alternately, techniques that are applied to a static configuration of soil are generally treated in batches where a batch or given amount of soil is introduced to the treatment apparatus; the treatment process is started, and when complete, is stopped and the treated soil removed. The next batch of soil is then introduced to the treatment apparatus. Static configuration techniques may also be broken down into two subcategories: (a) pile arrangement and (b) container arrangements.
Another characteristic of thermal desorption technologies is the source of heat and the gas used to effect the decontamination. The exact mechanism that occurs in thermal desorption is not well understood and a variety of techniques have been proposed in the prior art. Some processes use combustion gases from the burning of a fossil fuel as both the source of heat and the desorption gas. Sometimes the fuel is supplemented by recirculating the contaminated off-gas from the treated soil to the burn chamber as additional fuel. Other processes have used fresh air, or an inert gas, as the treatment gas, and heat the treatment gas indirectly in a heat exchanger prior to introducing it to the soil, or heat the soil and not the treatment gas.
Nearly all the prior art processes use combustion of fossil fuel as a heat source. This has the undesirable consequence of forming products of incomplete combustion, oxides of nitrogen, and other greenhouse gases as a by-product. Combustion also has the potential to add unburned hydrocarbons to the process exhaust gas if strict control of the combustion process is not maintained.
A variety of temperatures have been used for the treatment gas and in control of the off-gas temperature, which is indicative of the soil temperature. The temperature and time at temperature may be varied depending on the characteristics of the soil and its"" contaminates.
The prior art contains a variety of processes making use of all the above variables. U.S. Pat. No. 4,738,206 (Noland), U.S. Pat. No. 4,864,942 (Fochtman) and U.S. Pat. No. 4,977,839 (Fochtman) describe continuous process apparatus that make use of combustion gases for heat. These processes vary greatly in the temperatures used with U.S. Pat. No. 4,738,206 teaching a range of 120 to 450 degrees F. and U.S. Pat. Nos. 4,864,942 and 4,977,839 claiming a range of 300 to 400 degrees C. (which is 572 to 752 degrees F.) They also vary in the treatment gas with U.S. Pat. No. 4,738,206 using combustion gas and U.S. Pat. Nos. 4,864,942 and 4,977,839 teaching the use of an inert gas such as nitrogen, and the addition of water.
A characteristic of these continuous processes is the use of heavy material handling equipment that uses large amounts of energy in moving the material through the treatment process. This energy use is in addition to that expended in treating and in excavating the material and returning it to its final state.
Static processes that use a pile arrangement are described in U.S. Pat. No. 5,067,852 (Plunkett), U.S. Pat. No. 5,213,445 (Ikenberry), U.S. Pat. No. 5,228,804 (Balch), U.S. Pat. No. 5,836,718 (Price), and U.S. Pat. No. 6,000,882 (Bova). The apparatus of each of these consists of soil that is placed on a treatment surface then layered with differing configurations of piping until the desired configuration is attained. The pile is then covered with a vapor-proof covering prior to treatment. These processes also vary greatly in the temperature used. U.S. Pat. No. 5,067,852 uses unheated air as the treatment gas, but teaches that some heat is advantageous. At the other end of the temperature range U.S. Pat. No. 5,228,804 teaches the use of air heated in a heat exchanger to 1200 to 1400 degrees F. as a treatment gas. More moderate treatment gas temperatures, to 300 degrees F., are used in U.S. Pat. No. 5,213,445 using a treatment gas of combustion products from recirculating the off-gas, while U.S. Pat. No. 6,000,882 injects combustion gas of at least 800 degrees F. and perhaps as high as 2500 degrees F. to raise the soil temperature to the 212 to 350 degree F. range, then exhausts the off-gas through the same piping. Another approach is taken by U.S. Pat. No. 5,836,718 in that the soil is heated by conduction through the walls of the piping in the soil pile to a temperature of 90 to 250 degree C. (194 to 452 degree F.) and the fresh air treatment gas is not heated.
The pile arrangement processes do not require energy-intensive material handling during treatment, however they may be characterized as requiring labor-intensive setup and disassembly in the activity of layering the piping system within the soil pile and removing it following treatment, and also in covering and uncovering the completed pile.
Static processes that use a container arrangement are not as prevalent in the prior art. One example is U.S. Pat. Reissue No. 36,222 (O""Ham) that has the contaminated soil loaded into a tray-shaped treatment container, and then directs combustion heat and gases on the surface of the soil while the off-gas is removed from the bottom of the container. Temperatures are not given, but the inlet gas temperature may be assumed to be in the upper end of the treatment range. U.S. Pat. No. 6,296,815 (Walker) takes another approach. The soil is loaded into tall-insulated containers and then electric resistance heaters are inserted into the soil. The containers are moved into an insulated treatment vessel and the soil heated directly. The details of the process are not given.
The advantages of a static process using a container is the container can provide for ease of loading and unloading material, reducing labor when compared with pile arrangements, and it does not require high energy costs for material handling when compared with continuous processing arrangements. A disadvantage of these prior-art container arrangements is they require handling the soil to move it from the container in which it was placed after excavation, which presumably would be a dump truck hopper, load it into the treatment container for treatment, and then handle it again following treatment to put it back into the dump truck hopper disposition.
The review of the prior art summarized above indicates a need for an ex-situ static process that is labor efficient by requiring only a single soil handling step during excavation and then maintains the soil in the same container until it is returned to the site of disposition, is time and energy efficient in the treatment process, and is environmentally friendly by avoiding combustion in the treatment process and by using air temperatures below those conducive to forming oxides of nitrogen.
The present invention can be categorized as a thermal desorption technique applied to a static configuration of soil in a batch process using a container arrangement. The process is designed to use temperatures based on testing of samples of the contaminated soil to be treated. Contaminates may vary widely and therefore the temperatures used will vary to obtain an efficient remediation process for the contaminate of interest.
The treatment process for thermal desorption of hydrocarbon contaminants from excavated soil provides efficient contaminant removal by handling the soil in a thermally conductive treatment vessel that is contained in an insulated treatment chamber for treatment. The soil is treated with fresh air that is dried and electrically heated prior to introduction to the treatment chamber. Excavating the soil directly into the treatment vessel allows the treated soil to be returned to the final disposition site in the same vessel, minimizing soil handling
The treatment vessel consists of a floor, sides and ends to contain contaminated soil that remains exposed at the top of the vessel, and a gas exit pathway arranged at a predetermined location within the contaminated soil such that gases in the contaminated soil flow to the gas exit pathway.
The treatment chamber has an opening so the treatment vessel may be inserted or removed, an incoming air penetration to direct the incoming air to locations external to the treatment vessel and a gas exit pathway penetration arranged so the gases in the pathway exit the treatment chamber.
An air dryer, air blower and electric air heater are arranged such that the incoming air to the treatment chamber is dried and heated upon entering the treatment chamber.
A gas extraction blower directs the gases in the gas exit pathway penetration to exit the treatment chamber. This air is cooled prior to flowing through the blower.
The process flow path then is for dry, heated incoming air to surround the treatment vessel transferring heat to the contaminated soil through the treatment vessel floor, sides, and ends inducing the migration of contaminates through the soil to the gas exit pathway, and then the heated air then flows through the contaminated soil, directly heating the soil before entering the gas exit pathway and exiting the chamber.
One object of this invention is to provide a thermal desorption technique which uses a non-combustive heat source. This eliminates the formation of oxides of nitrogen due to combustion and the potential for addition of incomplete combustion products to the process exhaust.
A second object of this invention is to provide a treatment that controls the maximum temperature of the air, thereby maintaining temperatures below those conducive to formation of oxides of nitrogen.
A third object of this invention is to use dry air as the treatment gas. This increases the efficiency of heat delivery by eliminating the latent heat of vaporization of entrained moisture and improves the efficiency of hydrocarbon and water removal from the treated soil.
A fourth object of this invention is to provide efficient heat transfer to the soil. This reduces the energy required to treat a batch of soil.
A fifth object of this invention is to provide uniform heating of the soil so as to assure that all the soil receives a thorough treatment.
A sixth object of this invention is to provide a soil treatment container in which the soil may be inserted into as it is excavated, and therefore does not require further handling of the soil until treatment is complete and it is being placed in its final disposition location.
A seventh object of this invention is to provide a soil treatment process which is conducted in batches that allows adjusting the treatment parameters (time, temperature) for the specific soil and contaminates encountered at the treatment location.